energies
Article
Synthesis of Mesoporous γ-Alumina Support for Water Composite
Sorbents for Low Temperature Sorption Heat Storage
Manca Ocvirk
1,2
, Alenka Risti´ c
1,
* and Nataša Zabukovec Logar
1,3
Citation: Ocvirk, M.; Risti´ c, A.;
Zabukovec Logar, N. Synthesis of
Mesoporous γ-Alumina Support for
Water Composite Sorbents for Low
Temperature Sorption Heat Storage.
Energies 2021, 14, 7809. https://
doi.org/10.3390/en14227809
Academic Editor:
Elpida Piperopoulos
Received: 15 October 2021
Accepted: 19 November 2021
Published: 22 November 2021
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1
National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia; manca.ocvirk@ki.si (M.O.);
natasa.zabukovec@ki.si (N.Z.L.)
2
Jožef Stefan International Postgraduate School, Jamova Cesta 39, 1000 Ljubljana, Slovenia
3
School of Science, University of Nova Gorica, Vipavska Cesta 13, 5000 Nova Gorica, Slovenia
* Correspondence: alenka.ristic@ki.si; Tel.: +386-14760215
Abstract: The efficiency of thermochemical heat storage is crucially determined by the performance
of the sorbent used, which includes a high sorption capacity and a low regeneration temperature. The
thermochemical salt hydrate– γ-alumina composite sorbents are promising materials for this applica-
tion but lack systematic study of the influence of γ-alumina structural properties on the final storage
performance. In this study, mesoporous γ-Al
2
O
3
supports were prepared by solvothermal and
hydrothermal synthesis containing a block copolymer (F-127) surfactant to design thermochemical
CaCl
2
and LiCl composite water sorbents. Altering the solvent in the synthesis has a significant effect
on the structural properties of the γ-Al
2
O
3
mesostructure, which was monitored by powder XRD,
nitrogen physisorption, and SEM. Solvothermal synthesis led to a formation of mesoporous γ-Al
2
O
3
with higher specific surface area (213 m
2
/g) and pore volume (0.542 g/cm
3
) than hydrothermal
synthesis (147 m
2
/g; 0.414 g/cm
3
). The highest maximal water sorption capacity (2.87 g/g) and
heat storage density (5.17 GJ/m
3
) was determined for W-46-LiCl containing 15 wt% LiCl for space
heating, while the best storage performance in the sense of fast kinetics of sorption, without sorption
hysteresis, low desorption temperature, very good cycling stability, and energy storage density of
1.26 GJ/m
3
was achieved by W-46-CaCl
2
.
Keywords: mesoporous γ-Al
2
O
3
; TCM composite; hydrothermal synthesis; structural properties;
water sorption capacity; sorption heat storage
1. Introduction
Thermochemical energy storage (TCES), as one of the three technologies of thermal
energy storage (TES), can reduce fossil fuel consumption, which still dominates building
space heating, by shifting some of the solar thermal energy collected in summer to winter [1].
Thermochemical heat storage uses the reversible chemical reaction (reaction of water and
CaCl
2
) and/or sorption processes of gases in solids or liquids (water sorption on porous
solids). Water sorption heat storage consists of two phases. In the first phase, which is
also called charging or desorption, gas or water vapour is desorbed from the material
under solar radiation or waste heat. In the second phase or discharging (adsorption), the
water vapour is adsorbed on the material with heat release, which can be used for space
heating. The efficiency of the sorption technology, based on the alternating (ad)sorption
(exothermic phenomenon) and desorption (endothermic phenomenon) of the working fluid
on the sorption materials, is determined by the performance of the used sorbent, which
should have a high sorption capacity, and consequently a high energy storage density, a
low regeneration temperature of 80 to 120
◦
C, stability in humid conditions and also at
temperatures up to 120
◦
C, and no hysteresis (of the sorption and desorption curves) during
the sorption cycles. These requirements can be accomplished by the thermochemical (TCM)
composite sorbents [2], formed of various porous supports and hygroscopic salt hydrates
(halides, nitrates, sulphates) and are currently the most studied sorbents, especially when
Energies 2021, 14, 7809. https://doi.org/10.3390/en14227809 https://www.mdpi.com/journal/energies